Distributed optical fiber sensor based on roman and brillouin scattering

ABSTRACT

A distributed optical fiber sensor based on Raman and Brillouin scattering is provided. The distributed optical fiber sensor includes a semiconductor FP cavity pulsed wideband optical fiber laser ( 11 ), a semiconductor external-cavity continuous narrowband optical fiber laser ( 12 ), a wave separator ( 13 ), an electro-optic modulator ( 14 ), an isolator ( 15 ), an Er-doped optical fiber amplifier ( 16 ), a bidirectional coupler ( 17 ), an integrated wavelength division multiplexer ( 19 ), a first photoelectric receiving and amplifying module ( 20 ), a second photoelectric receiving and amplifying module ( 21 ), a direct detection system ( 22 ), a narrowband optical fiber transmission grating ( 23 ), a circulator ( 24 ) and a coherence detection module ( 25 ). The temperature and the strain can be measured simultaneously, and the signal-to-noise ratio of the system is enhanced.

TECHNICAL FIELD

The present invention relates to a technical field of optical fibersensing, and specifically to a distributed optical fiber sensor based onRaman and Brillouin scattering.

BACKGROUND OF THE INVENTION

In the field of distributed optical fiber sensors, there are distributedoptical fiber Raman scattering photon temperature sensors anddistributed Brillouin scattering photon sensors. In the Chinese patentCN101324424, the detection and pumping light source is adopted, anoptical fiber Raman amplifier is used to replace a traditional opticalfiber Brillouin amplifier to obtain backward optical fiber StimulatedBrillouin Scattering (SBS) light ray, and strain information is obtainedby measurement of the frequency shifts of the SBS light ray. However,the optical fiber Raman amplifier is expensive and high in cost. Using anarrowband laser light source, the Newson research team from Universityof Southampton in United Kingdom measures fiber temperature variationsby the detection of optical fiber backward spontaneous anti-Stokes Ramanscattering, and measures the fiber strains by the detection ofspontaneous optical fiber Brillouin scattering. However, as the opticalfiber Brillouin scattering has narrow spectrum bandwidth, themeasurement precision of temperatures or strains is low (M. N.Allahbabi, Y. T. Cho and T. P. Newson, Simulataneous DistributedMeasurements of Temperature and Strain using Spontaneous Raman andBrillouin Scattering, Optics Letters, 2005, 1 Jun., p. 1276-1278). TheChinese patent CN101162158 is suitable for measurement of thetemperature and strain of a super-remote optical fiber. However, anoptical fiber Raman amplifier is embedded into the system, which tendsto cause mutual interference due to non-linear effect of the opticalfiber. Moreover, the optical fiber Raman amplifier is expensive and highin cost.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides a distributedoptical fiber sensor based on Raman and Brillouin scattering.

One technical solution of the present invention is as follows: adistributed optical fiber sensor based on Raman and Brillouin scatteringincludes: a semiconductor Fabary-Perot (FP) cavity pulsed widebandoptical fiber laser, a semiconductor external-cavity continuousnarrowband optical fiber laser, a wave separator, an electro-opticmodulator, an isolator, an Er-doped optical fiber amplifier, abidirectional coupler, an integrated wavelength division multiplexer,two photoelectric receiving and amplifying modules, a direct detectionsystem, a narrowband optical fiber transmission grating, a circulator,and a coherence detection module; wherein an output terminal of thesemiconductor FP cavity pulsed wideband optical fiber laser is connectedwith an input terminal of the Er-doped optical fiber amplifier; anoutput terminal of the semiconductor external cavity continuousnarrowband optical fiber laser is connected with an input terminal ofthe wave separator; an output terminal of the wave separator isconnected successively to the electro-optic modulator, the isolator andanother input terminal of the Er-doped optical fiber amplifier; anoutput terminal of the Er-doped optical fiber amplifier is connectedwith an input terminal of the bidirectional coupler; an output terminalof the bidirectional coupler is connected with a single-mode opticalfiber, and another output terminal of the bidirectional coupler isconnected with an input terminal of the integrated wavelength divisionmultiplexer; two output ports of the integrated wavelength divisionmultiplexer are connected with the direct detection system via the firstphotoelectric receiving and amplifying module and the secondphotoelectric receiving and amplifying module respectively, and a thirdoutput port of the integrated wavelength division multiplexer isconnected with an input terminal of the circulator via the narrowbandoptical fiber transmission grating; another input terminal of thecirculator is connected with another output terminal of the waveseparator, and an output terminal of the circulator is connected withthe coherence detection module.

The above-mentioned semiconductor FP cavity pulsed wideband opticalfiber laser has a pulse width less than 30 ns and wavelength of 1550 nm.The semiconductor external-cavity continuous narrowband optical fiberlaser has the spectrum width of 10 MHz and wavelength of 1555 nm. Thetwo light sources are in different wavebands, thus achieving wavelengthdivision multiplexing.

The above-mentioned integrated wavelength division multiplexer includes:two pairs of optical fiber couplers, a self-focusing lens with parallellight path, an optical filter with the central wavelength of 1450 nm,the spectrum bandwidth of 38 nm and the attenuation less than 0.3 dB,and an optical filter with the central wavelength of 1660 nm, thespectrum bandwidth of 40 nm and the attenuation less than 0.3 dB; theintegrated wavelength division multiplexer has four ports including oneinput port and three output ports. The first output port is working at1450 nm for optical fiber anti-Stokes Raman scattering light, the secondoutput port is working at 1660 nm for optical fiber Stokes Ramanscattering light, and the third output port is working at 1550 nm foroptical fiber Rayleigh and Brillouin scattering light.

The above-mentioned narrowband optical fiber transmission grating is anoptical fiber grating with the central wavelength of 1555.08 nm, thespectrum bandwidth of 0.1 nm, the attenuation less than 0.3 dB, and theisolation higher than 35 dB.

The distributed optical fiber sensor based on Raman and Brillouinscattering according to embodiments of the present invention is based onthe wavelength division multiplexing theory and the fusion theory of theoptical fiber non-linear optical scattering, measures temperatures ofthe optical fiber based on an intensity ratio of the backward opticalfiber spontaneous anti-Stokes and Stokes Raman scattering light rays,and measures the strains of the optical fiber based on frequency shiftsof the backward optical fiber spontaneous Brillouin scattering light,thus realizing simultaneous measurement of the temperature and thestrain, enhancing the signal-to-noise ratio of the system, and improvingthe measurement precision.

The laser generated by the semiconductor FP cavity pulse widebandoptical fiber laser passes through the Er-doped optical fiber amplifierand the bidirectional coupler into the single-mode optical fiber, thebackward Raman scattering light from the single-mode optical fiberpasses through the bidirectional coupler and is input into theintegrated wavelength division multiplexer; the anti-Stokes and Stokesspontaneous Raman scattering light rays output from the first and secondoutput ports of the integrated wavelength division multiplexer passthrough the first and second photoelectric receiving and amplifyingmodules respectively, and enter into the direct detection system. Thedirect detection system processes the signals received, and provides thetemperature information of each segment of the optical fiber accordingto a power ratio value between the anti-Stokes and Stokes spontaneousRaman scattering light rays. The continuous laser output from thesemiconductor external cavity continuous narrowband optical fiber laserpasses through the wave separator and modulated by the electro-opticmodulator into a 30 ns pulse laser light, and then further passesthrough the isolator, the Er-doped optical fiber amplifier and thebidirectional coupler into the single-mode optical fiber; the backwardBrillouin scattering light of the optical fiber passes successivelythrough the bidirectional coupler, the third output port of theintegrated wavelength division multiplexer, the narrowband optical fibertransmission grating into the circulator, and is, together with thelocal laser from the wave separator into the circulator, input into thecoherence detection module. The frequency shift of the optical fiberBrillouin scattering light is measured by the coherence detectionmodule, and then the information of strain and temperature of eachsegment of the optical fiber is obtained.

The temperature measurement theory of optical fiber Raman scattering is:the intensity ratio I (T) of the anti-Stokes Raman scattering light tothe Stokes Raman scattering light:

$\begin{matrix}{{I(T)} = {\frac{\varphi_{a}}{\phi_{s}} = {\left\lbrack \frac{v_{a}}{v_{s\;}} \right\rbrack^{4}^{- {(\frac{h\; \Delta \; v_{T}}{kT})}}}}} & (1)\end{matrix}$

wherein φ_(a)

φ_(s) are electrical level values after photoelectric transformation,ν_(a), ν_(s) are frequencies of the anti-Stokes Raman scattering photonand the Stokes Raman scattering photon, h is the Planck constant andh=6.626 068 76.52×10⁻³⁴ J.s (the basic physics constant datum in year1998), Δν is the phonon frequency of the optical fiber molecule andequals to 13.2 THz, k is the Boltzmann constant and k=1.380 650324×10⁻²³JK⁻¹, T is the Kelvin absolute temperature. Based on the intensity ratioof the two types of light, temperature information of each segment ofthe optical fiber is obtained.

The theory for strain and temperature measurement of optical fiberBrillouin scattering is as follows: in the optical fiber, the incidentlaser in the optical fiber interacts non-linearly with the sound wave inthe optical fiber, the sound wave is then generated throughelectrostriction of the light wave, which causes periodical modulationto an optical fiber refractive index to form a spatial refractive indexgrating, generates Brillouin scattering light with frequency shifts; thefrequency shift ν_(B) of the backward Brillouin scattering formed in theoptical fiber is:

ν_(B)=2nv/λ  (2)

wherein n is the refractive index at the incident light wavelength λ, vis the sound velocity in the optical fiber; for the quartz opticalfiber, ν_(B) is about 11 GHz when λ is approximately 1550 nm.

The frequency shift ν_(B) of the Brillouin scattering light in theoptical fiber has strain and temperature effect:

$\begin{matrix}{v_{B} = {v_{B_{0}} + {\frac{\partial v}{\partial ɛ}{ɛ\left( {\mu \; ɛ} \right)}} + {\frac{\partial v}{\partial T}{T\left( {{^\circ}\mspace{14mu} {C.}} \right)}}}} & (3)\end{matrix}$

The frequency shift of the Brillouin scattering light is:

δν_(B) =C _(vε) δε+C _(vT) δT  (4)

wherein the strain coefficient C_(vε) and the temperature coefficientC_(vT) of the frequency shift are:

C _(vε)=0.0482±0.004 MHz/με, C _(vT)=1.10±0.02 MHz/K

The strain amount of each segment of the optical fiber is obtained bymeasurement of the frequency shift of the optical fiber Brillouinscattering light ray.

The technical advantages of the present invention include: according toembodiments of the present invention, based on the fusion theory and thewavelength division multiplexing theory of the optical fiber non-linearoptical scattering, two laser light sources are adopted. One is thesemiconductor FP cavity pulsed wideband optical fiber laser, andmeasures temperature based on the intensity ratio of the optical fiberspontaneous Raman scattering; the other is the semiconductorexternal-cavity continuous narrowband optical fiber laser, and measuresstrain based on the frequency shift of the optical fiber spontaneousBrillouin scattering light ray. Thereby, the signal-to-noise ratio ofthe system is enhanced, the temperature and the strain can be spacialymeasured simultaneously, and the measurement precision can also beimproved.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a structure of a distributedoptical fiber sensor based on Raman and Brillouin scattering, whichillustrates a semiconductor FP cavity pulsed wideband optical fiberlaser 11, a semiconductor external-cavity continuous narrowband opticalfiber laser 12, a wave separator 13, an electro-optic modulator 14, anisolator 15, an Er-doped optical fiber amplifier 16, a bidirectionalcoupler 17, an integrated wavelength division multiplexer 19, a firstphotoelectric receiving and amplifying module 20, a second photoelectricreceiving and amplifying module 21, a direct detection system 22, anarrowband optical fiber transmission grating 23, a circulator 24 and acoherence detection module 25.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, the distributed optical fiber sensor based on Ramanand Brillouin scattering in the present invention includes: asemiconductor FP cavity pulsed wideband optical fiber laser 11, asemiconductor external-cavity continuous narrowband optical fiber laser12, a wave separator 13, an electro-optic modulator 14, an isolator 15,an Er-doped optical fiber amplifier 16, a bidirectional coupler 17, anintegrated wavelength division multiplexer 19, a first photoelectricreceiving and amplifying module 20, a second photoelectric receiving andamplifying module 21, a direct detection system 22, a narrowband opticalfiber transmission grating 23, a circulator 24, and a coherencedetection module 25.

In an embodiment, an output terminal of the semiconductor FP cavitypulsed wideband optical fiber laser 11 is connected with an inputterminal of the Er-doped optical fiber amplifier 16; an output terminalof the semiconductor external-cavity continuous narrowband optical fiberlaser 12 is connected with an input terminal of the wave separator 13;an output terminal of the wave separator 13 is connected successively tothe electro-optic modulator 14, the isolator 15 and another inputterminal of the Er-doped optical fiber amplifier 16; an output terminalof the Er-doped optical fiber amplifier 16 is connected with an inputterminal of the bidirectional coupler 17; an output terminal of thebidirectional coupler 17 is connected with a single-mode optical fiber18, and another output terminal of the bidirectional coupler 17 isconnected with an input terminal of the integrated wavelength divisionmultiplexer 19; two output ports of the integrated wavelength divisionmultiplexer 19 are connected with the direct detection system 22 via thefirst photoelectric receiving and amplifying module 20 and the secondphotoelectric receiving and amplifying module 21 respectively, and athird output port of the integrated wavelength division multiplexer 19is connected with an input terminal of the circulator 24 via thenarrowband optical fiber transmission grating 23; another input terminalof the circulator 24 is connected with another output terminal of thewave separator 13, and an output terminal of the circulator 24 isconnected with the coherence detection module 25.

The semiconductor pulsed optical fiber laser 11 is a semiconductor FPcavity high power optical fiber laser having a pulse width less than 30ns and central wavelength of 1550 nm. The semiconductor external-cavitynarrowband optical fiber laser 12 is a semiconductor external-cavitycontinuous optical fiber laser 12 having a spectrum width of 10 MHz andthe central wavelength of 1555 nm, and is modulated, by theelectro-optic modulator 14, to a pulsed laser having a pulse width of 30ns. The two light sources are in different wavebands, and achievewavelength division multiplexing.

The integrated wavelength division multiplexer 19 may be a SZMX-WDM-2type wavelength division multiplexer from Ming Xin PhotoelectricCompany, which includes: two pairs of optical fiber couplers, aself-focusing lens with parallel light path, an optical filter withcentral wavelength of 1450 nm as well as spectrum bandwidth of 38 nm andattenuation less than 0.3 dB, and an optical filter with the centralwavelength of 1660 nm, the spectrum bandwidth of 40 nm and theattenuation less than 0.5 dB. The integrated wavelength divisionmultiplexer 19 has one input port and three output ports, wherein thefirst output port is working at 1450 nm, the second output port isworking at 1660 nm and the third output port is working at 1550 nm, andwherein the first output port is for optical fiber anti-Stokes Ramanscattering light, the second output port is for optical fiber StokesRaman scattering light, and the third output port is for optical fiberRayleigh and Brillouin scattering light.

The narrowband optical fiber transmission grating 23 may be a narrowbandoptical fiber transmission grating with central wavelength of 1555.08nm, or may be an optical fiber grating with spectrum bandwidth of 0.1nm, attenuation less than 0.3 dB and isolation more than 35 dB. Thenarrowband optical fiber transmission grating 23 selects the opticalfiber Brillouin scattering light from the third output port of theintegrated wavelength division multiplexer and isolates the backwardoptical fiber Rayleigh scattering light.

Either of the first photoelectric receiving and amplifying module 20 andthe second photoelectric receiving and amplifying module 21 is formed ofa low-noise InGaAs photoelectric avalanche diode, a low-noise MAX4107preamplifier and a main amplifier which are connected by optical fibers.

The direct detection system 22 may be an NI5911 type signal processingcard with dual-channel bandwidth of 100 MHz and a collection rate of 100MS/s from U.S.A. NI (National Instruments) Inc., or may be a CS21GB-1GHz type signal processing card with dual channels and a collection rateof 500 MS/s from Canada GaGe Inc.

The coherence detection module 25 performs coherence detection on thebackward optical fiber Brillouin scattering light and the local light ofthe external-cavity narrowband optical fiber laser by means of beatfrequency performed by a photoelectric detector, and measures thefrequency shift to obtain the strain information of each segment of theoptical fiber.

1. A distributed optical fiber sensor based on Raman and Brillouinscattering, characterized by comprising: a semiconductor FP cavitypulsed wideband optical fiber laser (11), a semiconductorexternal-cavity continuous narrowband optical fiber laser (12), a waveseparator (13), an electro-optic modulator (14), an isolator (15), anEr-doped optical fiber amplifier (16), a bidirectional coupler (17), anintegrated wavelength division multiplexer (19), a first photoelectricreceiving and amplifying module (20), a second photoelectric receivingand amplifying module (21), a direct detection system (22), a narrowbandoptical fiber transmission grating (23), a circulator (24), and acoherence detection module (25); wherein an output terminal of thesemiconductor FP cavity pulsed wideband optical fiber laser (11) isconnected with an input terminal of the Er-doped optical fiber amplifier(16); an output terminal of the semiconductor external-cavity continuousnarrowband optical fiber laser (12) is connected with an input terminalof the wave separator (13); an output terminal of the wave separator(13) is connected successively to the electro-optic modulator (14), theisolator (15) and another input terminal of the Er-doped optical fiberamplifier (16); an output terminal of the Er-doped optical fiberamplifier (16) is connected with an input terminal of the bidirectionalcoupler (17); an output terminal of the bidirectional coupler (17) isconnected with a single-mode optical fiber (18), and another outputterminal of the bidirectional coupler (17) is connected with an inputterminal of the integrated wavelength division multiplexer (19); twooutput ports of the integrated wavelength division multiplexer (19) areconnected with the direct detection system (22) via the firstphotoelectric receiving and amplifying module (20) and the secondphotoelectric receiving and amplifying module (21) respectively; a thirdoutput port of the integrated wavelength division multiplexer (19) isconnected with an input terminal of the circulator (24) via thenarrowband optical fiber transmission grating (23); another inputterminal of the circulator (24) is connected with another outputterminal of the wave separator (13), and an output terminal of thecirculator (24) is connected with the coherence detection module (25).2. The distributed optical fiber sensor according to claim 1, whereinthe semiconductor FP cavity pulsed wideband optical fiber laser (11) haspulse width less than 30 ns and wavelength of 1550 nm.
 3. Thedistributed optical fiber sensor according to claim 1, wherein thesemiconductor external-cavity continuous narrowband optical fiber laser(12) has spectrum width of 10 MHz and wavelength of 1555 nm.
 4. Thedistributed optical fiber sensor according to claim 1, wherein theintegrated wavelength division multiplexer comprises: two pairs ofoptical fiber couplers, a self-focusing lens with parallel light path,an optical filter with central wavelength of 1450 nm, spectrum bandwidthof 38 nm and attenuation less than 0.3 dB, and an optical filter withcentral wavelength of 1660 nm, spectrum bandwidth of 40 nm andattenuation less than 0.3 dB; and wherein the integrated wavelengthdivision multiplexer has four ports including one input port and threeoutput ports, wherein a first output port is working at 1450 nm foroptical fiber anti-Stokes Raman scattering light, a second output portis working at 1660 nm for optical fiber Stokes Raman scattering light,and a third output port is working at 1550 nm for optical fiber Rayleighand Brillouin scattering light.
 5. The distributed optical fiber sensoraccording to claim 1, wherein the narrowband optical fiber transmissiongrating (23) is an optical fiber grating with central wavelength of1555.08 nm, spectrum bandwidth of 0.1 nm, attenuation less than 0.3 dB,and isolation higher than 35 dB.